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For the fast gear set, the gear ratio are: 33/21, 35/21. The original gear are 37/21, it will be more fast than the original gear ratio setting. For the original setting, pinion gear ratio are 16, 18 and 20T, use with 35T spur gear, pinion gear ratio are change 18, 20 and 22T, for use with 33T spur gear, pinion gear ratio are 20, 22 and 24T. Fast gear set provide a high speed for your m-chassis.